At present, telephone traffic is controlled either by electromechanical or by electronic exchanges. As concerns call signals sent from such exchanges to the subscribers, significant differences exist. Thus, electromechanical exchanges generally send a call signal with a rather high voltage level (of the order of 100 volts) at a relatively low frequency of 25 Hz, this signal being capable of directly actuating a conventional tone ringer of electromechanical type. Electronic exchanges, on the other hand, generally emit a voice-band call signal whose frequency range is 400 to 500 Hz and whose voltage level is very low (of the order of a few volts); hence, the latter signal cannot directly excite an electromechanical tone ringer.
Consequently, in telephone sets connected to electronic exchanges, a suitable electronic tone ringer is provided. Such a tone ringer, however, is not actuatable by the 25-Hz call tone of electromechanical exchanges.
If telephone sets were equipped with tone ringers compatible with both types of call signals referred to, expensive modifications in the exchanges and/or in associated telephone sets could be avoided when an electromechanical exchange is replaced by a new electronic exchange.
A number of tone ringers are known in the art which, for operating in both cases, require that the call signal coming from the exchange be accompanied by a polarity inversion of the d-c voltage present on the subscriber's line loop; to energize such a tone ringer, either the call signal used in traditional exchanges or a d-c voltage present on the line loop can be utilized, provided the polarity is inverted upon the arrival of a voice-band call signal from an electronic exchange.
Obviously, this polarity inversion entails higher costs and increased circuit complexity in the telephone exchange.
British Pat. No. 1,245,813 describes a tone ringer utilizing a particular electronic circuit designed to act as a selective amplifier for call signals coming from electronic exchanges and as an oscillator for call signals coming from electromechanical exchanges.
As appears from the description of that patent, the entire tone-ringer circuit is permanently energized by the subscriber's loop and no means are provided to limit current absorption in the absence of a call signal. Moreover, the circuit operates, in the absence of a call signal, as an amplifier permanently inserted in the subscriber's loop, and thus it continuously amplifies all the noises of various nature that may be present, thereby producing in the associated electro-acoustic transducer continuous series of rings and sounds which are very annoying.
Another inconvenience of this prior system is the dissimilar response of the circuit to the two types of calls. In the presence of a call coming from an electromechanical exchange, a sound obtained by the modulation at 25 Hz of the normal operating frequency of the local oscillator is produced; on the other hand, in the presence of a call signal coming from an electronic exchange a nonmodulated sound is generated, unless such modulation is performed in the exchange on the call signal.